Fig. 4.

The binary quantum spin-1/2 correlations ${\mathcal{Q}}_{1/2}$, which happens to be the set of trigonometric polynomials $P(+|\theta )=c_0+c_{1}cos\theta +s_{1}sin\theta$ with $0\le P(+|\theta )\le 1$ for all $\theta$. The two endpoints are the constant zero and one functions, and the other extremal points on the circle correspond to functions $\theta \mapsto \frac{1}{2}+\frac{1}{2}cos(\theta -\phi )$, with $\phi$ some fixed angle